This invention relates generally to gas turbine engines and more particularly to an improved fan outlet structural guide vane.
A gas turbine engine, such as a turbofan engine for an aircraft, includes a fan section, a compression section, a combustion section and a turbine section. The engine has a centrally located axis, which extends longitudinally through the sections. A fan case and a smaller diameter compressor case cooperate to radially bound an annular fan duct and divide the flow path for the working medium gases into a primary flow path and a secondary flow path. The primary flow path extends axially through the sections of the engine. The secondary flow path extends through the fan duct parallel to and radially outward of the primary flow path.
During engine operation, the fan draws the working medium gases, for example air, into the engine. The air drawn along the primary flow path into the compression section is compressed. The compressed air is channeled to the combustion section where fuel is added to the compressed air and the air/fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressor. Additional energy not needed to drive the fan and compressor contributes to useful thrust.
In traditional turbofan engines, rigid radial struts extend between the fan case and the compressor case to support static and dynamic loads during operation. This assembly of the fan case, compressor case, and radial struts is frequently referred to as the intermediate casing. A fan outlet guide vane is situated upstream from the struts of the intermediate case and downstream from the fan to de-swirl the circumferentially flowing air discharged from the fan into an axial direction prior to the air flowing into the primary and secondary flow paths. As there is always a need in the aircraft industry to reduce engine weight and complexity, some turbofan engines now replace the radial struts and outlet guide vane with a single array of structural guide vanes that function to both de-swirl the airflow exiting the fan and support static and dynamic loads during engine operation.
This dual function structural guide vane generally includes an assembly of airfoils extending between the fan case and the compressor, and circumferentially spaced apart from one another. In some prior art embodiments, the end of each airfoil includes a platform that would abut against the platforms of adjacent airfoils to form the aerodynamic outer diameter and inner diameter of the flow path across the structural guide vane. Generally, the outer platform of each airfoil is connected to the fan case by bolts, and the inner platform of each airfoil is connected to the compressor case by an assembly of bolts, spacers, nuts and inserts. Because the outer platform of each airfoil is situated between the fan case and its corresponding airfoil, the outer platform transfers loads from the airfoil to the fan case. Similarly, the inner platform of each airfoil transfers loads from the compressor case to its corresponding airfoil due to its being situated between the airfoil and the compressor case. Due to the loads they experience during engine operation, the outer platforms and the inner platforms must be reinforced to support the loads and resist bending and deformation.
Prior art methods of reinforcing the platforms include forming a large number of relatively tall axial ribs and circumferential ribs on the platform sides opposite the airfoil. Using a large number of tall axial ribs and circumferential ribs on each platform does strengthen the platforms against bending, but it does so at the cost of adding to the overall weight of the engine. Prior art methods also include coupling the ends of each airfoil directly to the fan case and compressor, and assembling platform panels between the airfoils to create the outer diameter and inner diameter flow paths across the structural guide vane. By connecting the airfoils directly to the cases, the platform panels do not require reinforcement as they do not transfer loads between the compressor case, the airfoils, and the fan case. However, the platform panels do increase the overall complexity and cost of the engine by increasing the number of parts to assemble during the initial assembling and in the event that the structural guide vane requires disassembly for maintenance or repair.